ATT System 25 Reference Manual

							Data Services Overview
Data Services Overview
System 25’s data features provide switched data transmission at up to 19,200 bps (RS-232
interface), and a 212A modem compatible conversion resource capable of handling data at
300 and 1200 bps.
The system provides switched connections between data endpoints. These endpoints
include data terminals, personal computers, multiport computers, and modems.
Data
endpoints are either digital data endpoints or analog data endpoints.
Analog endpoints are connected to System 25 voice terminal or trunk port circuits through a
modem in the traditional manner. Digital endpoints are connected to System 25 data port
circuits on the TN726 Data Line CP. An Asynchronous Data Unit (ADU) is required in place
of the modem used with analog endpoints. Section 4 of this manual shows the connections
supported and required connecting equipment.
Data calls can be set up between data endpoints.Analog to analog and digital to digital
connections are straightforward; calls between analog and digital endpoints are possible only
if the system is equipped with a conversion resource (TN758 Pooled Modem Circuit Pack or
external modem pool). System 25 data calls from analog endpoints (including those to digital
endpoints) are set up in the traditional manner.The calling party should follow the
procedures supplied with his/her modem. However, a Modem Request Code must be dialed
when calling a digital endpoint.
Call setup from digital endpoints is facilitated by several data features: Command Mode,
Expert Mode, Data Terminal Dialing, Modem Pooling, Third-Party Call Setup, and Transfer To
Data.
In the discussion that follows, it is important to understand the difference between analog
voice terminology and data terminology. Refer to the “Glossary” (Section 9).
The following provides a definition of a data call in terms of its contextual components. The
components are (1) data endpoints, (2) data endpoint states, (3) data call processing modes,
(4) connecting configurations, and (5) controlling features.
Data Endpoints
Data endpoints are composed of data terminal equipment, an ADU or modem, and a
connection to the switch via an analog or data port. A digital data endpoint is addressed by
its Data Dial Code (DDC). Analog data endpoints are addressed like other voice terminals,
by their PDCs. For the remainder of this description, data endpoints will refer to digital data
endpoints unless stated otherwise.
Several different categories of data endpoints are supported. The categories have been
divided into two general groups, those having a DTE type interface, which encompasses
almost all of the data terminal devices, and a group of DCE interface devices (primarily
modems). The groups have then been divided into categories based upon their functional
attributes. However, it must be noted that within each category, control interfaces may vary.
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The following describes the categories and attributes of each:
1.DTE Devices
This group of data endpoints have one thing in common: their interface configuration
(although RS-232 control signal utilization varies significantly from terminal to
terminal). Some data equipment do not use any RS-232 control signals; these
require only BA (Transmitted Data Ready-Tx), BB (Received Data Ready-Rx) and AB
(Signal Ground) to function,while others require more RS-232 control signals to
operate. An ADU (Figure 2-21) can send Data Terminal Ready (DTR) from the data
terminal to the Data Line circuit and the Data Line circuit can send a control signal to
the data terminal. The signals Data Set Ready (DSR), Clear To Send (CTS), and
Received Line Signal Detector (DCD) are all connected to the control signal from the
DLC in the ADU and available if required by the data terminal. Refer to Section 5
(Port Specifications) for additional information.
TO/FROM
RS-232C
DTE
TD
RDADUTO/FROM
DTR(DATATN-726DCDMODULE)DATA LINEDSRCIRCUIT PACK
CTS(4-WIRE
CONNECTION)
* CD CONTROL SIGNAL CONNECTEDIN ADU TO PROVIDE CTS,
DSR, AND DCD TO RS-232CDEVICE
Figure 2-21.Asynchronous Data Unit Interface Signals
The following categories are part of the DTE data endpoint group:
a.Data Terminal Without ASCII Keyboard
This category includes such devices as Fax machines, EBCDIC or Baudot
terminals, and receive only devices such as printers.
Once connected to an ADU and turned on, these data endpoints appear on-
line, available, and ready to enter the Setup mode on auto-answered calls
(modes are described below).These endpoints will display or print
information received after a valid connection has been established without
additional RS-232 control from that endpoint.Note that since these
endpoints cannot establish calls for themselves, they must either be called by
other endpoints or have calls established for them via the Transfer to Data or
Third-Party Call Setup feature.
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b. ASCII Data Terminal With Keyboard
This category can be subdivided into two classes: (1) basic terminals, and (2)
intelligent, programmable data equipment such as personal computers. Basic
terminals appear to the data port to be on-line and available whenever they
are turned on, thus ready to enter either the Data Terminal Dialing mode. or
to enter the Setup mode on calls originating from a voice terminal or on
auto-answered calls.
In the case of most personal computers, a communications program must be
executed in order for it to communicate with its own RS-232 port or built-in
modem. Once the communications software is running, further operation will
be similar to that of the basic terminal.
c.Host Computer Endpoint
A host computer endpoint is very similar to a data endpoint with keyboard
except that the host has many ports and the interface is usually capable of
supporting multiple speeds and more of the RS-232 control signals. Front-
end communication software running in the host is typically supplied by the
computer vendor and is not designed to support the Data Terminal Dialing
feature. Such software typically supports call origination through Automatic
Calling Units (ACUs), which are not compatible with Data Terminal Dialing.
Thus, the primary means of communicating with the host is by calling from
data terminals or personal computers.
Groups of host ports with matching
characteristics may be members of hunt groups (referred to as host port
groups).
d. Analog Data Endpoint
Data endpoints with modems are referred to as analog data endpoints.
Modems connected via tip ring lines use PDCs as extension numbers rather
than Data Dial Codes (DDCs). Station-to-station data calls to (or from) this
endpoint from (or to) digital endpoints require a modem conversion resource
to convert the endpoint’s analog data to digital format. Calls from a digital
endpoint to an analog data endpoint (i.e., calls to a PDC), will automatically
have a conversion resource inserted in the calling path. If the called (analog)
endpoint should then invoke Transfer To Data, the conversion resource will
be released. Data calls originating from an analog data endpoint must first
enter a Modem Request Code before addressing a digital data endpoint.
This is required because the system assumes that a call originating from a
voice terminal will invoke Transfer To Data. If the originating station is not
going to transfer to data, it must indicate this so that a conversion resource
will be included in the connection.
2. DCE Devices
This group of data endpoints consists primarily of modems. The modems are
connected to a data port from their RS-232 side. The modem must be configured as
a DTE interface to connect to a System 25 data port. It is possible to simulate a DTE
interface from a modem with a cross-over (“null modem”) cable. This group of
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endpoints is important for users who provide their own modems, connected to
dedicated trunks or private lines, for internal modem pooling.
Data Endpoint States
The data endpoint may take on three states: (1) off-line, (2) on-line (on-hook), and (3) on-line
(off-hook). Off-line is when a data terminal is out of service (turned off, disconnected, etc,).
The on-line (on-hook) state occurs when the terminal is turned on, is available to answer a
call, but is not on a data call. Finally, the on-line (off-hook) state occurs when the data
endpoint is actively on a data call.
Data Call Processing Modes
Data calls differ both in signaling and call setup from voice calls. For this reason, a unique
set of data call processing modes have been defined to support data call operation in a
manner consistent with the characteristics of data terminals.
a.
b.
c.
d.
Off-Line Mode
The data endpoint is considered to be in the Off-Line Mode whenever the data
endpoint’s DTR signal is inactive (e.g.,“turned off”). The endpoint is considered
unavailable and calls to this endpoint will receive the “RINGING” message or
Ringback (indefinitely).
Idle Mode
The Idle Mode indicates that the data endpoint is in its on-line, on-hook state. While
idle, call processing will allow the endpoint to:
— Enter either Data Terminal Dialing mode to originate a data call, or enter the
Setup mode after a call is originated from a voice terminal (Transfer To Data)
or other data terminal (Third-Party Call Setup).
— Autoanswer a data call and go into Setup mode.
The data endpoint remains in the Idle mode while the user is establishing a data call
from a voice terminal until Transfer To Data is activated.
Command Mode
Command Mode enables the Data Terminal Dialing feature, allows the user to view
and change associated data port options, and provides access to the Third-party Call
Setup feature. Command Mode may be entered by going on-line and pressing Break
or Break-Return.
Expert Mode
Expert Mode is an enhancement to the Command Mode feature that provides an
alternative method of performing the full range of Command Mode functions. By
eliminating the display of menus and allowing multiple commands to be entered on a
single line, Expert Mode lends itself to computer-driven instructions. Individual users
who are very familiar with Command Mode operations may also find it useful.
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e.Data Terminal Dialing Mode
Data Terminal Dialing is a data feature accessed via Command Mode. It provides a
procedure to establish data calls without the use of a voice terminal. Data Terminal
Dialing supports both on-premises and off-premises data calls (with the support of
the System 25 Modem Pooling feature).Dialed digits are entered from the data
terminal keyboard or host computer (using a program compatible with Data Terminal
Dialing protocol). Call progress text messages are sent to the terminal in place of
call progress tones.Upon completion of digit entry, Data Call Setup mode is
entered.
f.Data Call Setup Mode
Data Call Setup Mode is a transitional state entered after Transfer To Data, Data
Terminal Dialing, or during auto-answer; it exists during the handshake between data
ports.
If the endpoints are compatible and handshaking is successful, a data connection is
established. If handshake failure occurs, the user is notified and the data endpoint
returns to the Idle mode. Successful handshake must occur within 15 seconds of
answer at the called data endpoint. This implies that the voice terminal user must
invoke Transfer To Data within 15 seconds after far-end answer. Similarly, if an
originating voice user calls a voice terminal and both users transfer to data, both
ends must transfer within the 15 second time limit.
If the data endpoint is optioned for Command Mode permission, the data endpoint
will receive call progress text messages while in the Data Call Setup mode.
g.Data Mode
Data Mode is first entered after successful completion of Data Call Setup.
Transparent communication between connected endpoints is provided in Data Mode.
Connecting Configurations
Refer to “Connectivity” in Section 4 for data equipment connections.
Controlling Features
It is possible to originate data calls from either a voice terminal with a Transfer to Data
button or from data endpoints that support Command Mode (i.e., ASCII data terminals with
keyboards and host computers).Several controlling features are provided to allow data
endpoints and voice terminals to set up data calls. The following briefly describes the Data
Service features used in controlling data calls:
a. Command Mode/Expert Mode
Command Mode and Expert Mode provide an interface to the Data Terminal Dialing
feature, the Third-Party Call Setup feature, and permits users to display and change
data port options.
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b. Data Terminal Dialing
Data Terminal Dialing provides call setup from terminals and host computers
c. Transfer To Data
Transfer To Data is the preferred method of data call origination from multiline voice
terminals equipped with Transfer to Data (DATA) buttons and associated digital data
endpoints. The DATA button is associated by DDC with a near end data endpoint. A
unique DATA button must be provided for each DDC that the voice terminal is
capable of controlling.Associated with each DATA button is an LED that reflects the
status of data endpoints as follows:
l Dark—Data endpoint is idle
l Winking
—Data endpoint is reserved (preindicated)
l Flashing—
Data endpoint is being alerted of an incoming call
l On Steady—Data endpoint is either in the on-line (off-hook) state or is
reserved for another user and busy.
Refer to the following feature descriptions for additional information:
l Command Mode
l Data Call Setup
l Data Terminal Dialing
l Expert Mode
l Modem Pooling
l Third-Party Call Setup.
l Transfer To Data
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Data Terminal Dialing
Description
This feature allows users to onginate (place) data calls from a data terminal. Voice terminal
dialing and call progress tones are replaced by keyboard dialing and call progress messages.
The message “DIAL:”’ prompts the user to enter the called number from the keyboard, and
“RINGING” or “DIALING ... COMPLETED” informs the user that the dialed number is being
called.
Table 2-1 provides a list of the call progress messages.
Note:The following dialing procedures assume that “Command Mode” is active,
Refer to the feature description on“Expert Mode” for an alternative method
of dialing.
Dialed Characters:
In addition to digits and the
“ # ” and “ V ” characters on the touch-tone pad, the dialed
number may contain the following special characters:
l The characters “(” “)” “—”and SPACE may be used to improve legibility. These
characters are ignored.
l The “%” or “,”characters may be used to cause a 1.5-second pause in dialing.
Multiple pause characters can be used to produce longer pauses.
l The “$” (mark) character indicates that the remaining digits are for end-to-end
signaling.
l UNDERSCORE or BACKSPACE characters may be used to correct previously typed
characters on the same line.
l The “@” character may be used to delete the entire line and start over with a new
DIAL: prompt.
Each line of dialing information may contain up to 27 characters. Note that all of the dialing
information, including pauses and ignored characters,must be typed on a single line
following the DIAL: prompt and terminated by Return.
Dialing Correction:
The backspace character (BS key or Ctrl-H keys) or underscore (“_”) may be used to cancel
the previously entered character. More than one entered character may be deleted by using
multiple backspace or underscore characters. The “@” character may be used to delete the
entire line of entered characters.
Pause:
To assist the completion of off-premises calls, the pause characters “%” or “,” may be
used. A pause character may be used to help ensure the receipt of dial tone before
continuing to dial. Each “%” or “,”causes a fixed delay of one and one-half (1.5) seconds.
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Pause characters may be used consecutively if a longer pause is required. Note that System
25 cannot detect tones, such as a second dial tone for end-to-end signaling.
End-to-End Signaling:
Data connections to off-premises destinations require that a conversion resource (pooled
modem) be inserted into the connection.Occasionally it is necessary to send additional
tones to the remote endpoint after the connection is established to signal the remote
equipment. A “mark” character ($) must be included on the DIAL: line to indicate to call
processing that the remaining digits are to be sent to the far end prior to insertion of the
conversion resource into the connection.The “mark” character marks the boundary
between the digits dialed to reach a distant endpoint, and the digits used by that distant
endpoint after it has answered.Pause characters may and usually should follow a “mark”
character. An example using a“mark” character and several pause characters is shown
below. Dashes are included for readability.
Examples of dialing are as follows:
l DIAL: 3478
l DIAL: 9-1-(201)-946-8123,,$,5678
l DIAL: 9%946-8123%%$%5678%137%110
Call Disposition:
Call progress messages corresponding to call progress tones provided to voice terminals are
listed in Table 2-1. The message supplied (indicating reorder, busy, ringback) depends on the
disposition of the call.
1.
2.
3.
When ringback is received the displayed message is“RINGING” (internal calls only).
For outside calls, the corresponding call progress message is
“DIALING . . . .”.
If the endpoint answers, the displayed message is“ANSWERED” (internal calls only).
Then, if the handshake succeeds, a data connection is established. For outside calls,
when the system has finished dialing, the message “COMPLETED” is displayed.
If the handshake fails because a connection cannot be established between
endpoints (e.g., a port optioned at 9600 baud attempts to talk to a conversion
resource that can only talk at 300 or 1200 baud), the user receives “INCOMPATIBLE
FAR END,” “DISCONNECTED,” and the data endpoint goes on-hook.
If the far end does not answer, the caller must press Break to terminate the call attempt.
If the disposition of the call is such that TRY AGAIN or BUSY (indicating reorder or intercept
and busy respectively) is received, the switch sends“DISCONNECTED” to the data terminal
and returns the data endpoint to idle mode.
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Table 2-1.
Call Progress Messages for Data Terminal Dialing
Displayed
MessageApplication
DIAL:Placing a call
RINGINGPlacing a call
BUSYPlacing a call
ANSWEREDPlacing or
receiving a call
TRY AGAINPlacing a call
INCOMING CALL-*Receiving a call
PLEASE ANS-Placing a call from
a voice terminal
DISCONNECTED*Call is terminated
CONNECTED,Call is connected
SPEED = NNNN
INCOMPATIBLEPlacing a call
FAR END
DIALING . .Placing a call
COMPLETED
PLEASE WAITPlacing a call
BAD NUMBERPlacing a call
NO MODEMPlacing a call
SESSION 1Placing or
terminating a call
Meaning
Equivalent to dial tone. Enter any required
facility number followed by the dialed
number and a RETURN.
Equivalent to Ringback Tone. Called
number (far-end) is being signaled.
Provided on internal calls only.
Equivalent to busy tone. Called number is
in use, or out of service. Provided on
internal calls only.
Notifies calling and called users that call
has been answered. Provided on internal
calls only.
Equivalent to Reorder Tone. System
facilities are currently not available or
invalid number.
Equivalent to ringing.
Originating voice terminal user has
transferred call to data terminal using
Transfer to Data.
Call or call attempt is disconnected.
Notifies user that the call connection is
established and what the baud rate is.
[Provided that “Connection indication”
(Data Port Action 77) is enabled.]
Notifies user that the handshake between
data end points has failed
Indicates off-premises call is being dialed
and that dialing is completed.
Call queued.
Bad dialed number
No modem available for a call that
requires one.
Specifies the session number (1) of the
data call to the calling party
* Bell sounds when message is displayed.
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Answering Endpoint:
When the dialed endpoint is alerted, the user receives “INCOMING CALL-”. (The called
terminal will auto-answer if it is turned on.) If the handshake succeeds, a data connection is
established and the “CONNECTED” message is displayed if so optioned. If the handshake
fails, the user receives “INCOMPATIBLE FAR END, DISCONNECTED” and the data endpoint
returns to idle mode.
Considerations
Data Terminal Dialing allows users to place data calls from their terminals using the Data
Terminal Dialing feature and allows users to review the options administered for their data
ports.
Interactions
The following features interact with Data Terminal Dialing
End-To-End Signaling: (See preceding text.)
Modem Pooling: Data calls between analog and digital endpoints require that a conversion
resource (TN758) be available.If one is not, the “NO MODEM” followed by “TRY AGAIN”
message will be displayed.
Speed Dialing:System Speed Dialing codes can be dialed from data terminals. Personal
Speed Dialing is not supported.
Administration Requirements
Data Port: See the table of Permissible Data Port Options in the “Command Mode” feature
description.
Hardware Requirements
TN726 Data Line CP to support each digital endpoint.
TN758 Pooled Modem CP to support data calls between digital and analog endpoints.
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